Lithography projection objective, and a method for correcting image defects of the same

ABSTRACT

A lithography projection objective for imaging a pattern to be arranged in an object plane of the projection objective onto a substrate to be arranged in an image plane of the projection objective comprises a multiplicity of optical elements that are arranged along an optical axis of the projection objective. The optical elements comprise a first group, following the object plane, of optical elements, and a last optical element, which follows the first group and is next to the image plane and which defines an exit surface of the projection objective and is arranged at a working distance from the image plane. The projection objective is tunable or tuned with respect to aberrations for the case that the volume between the last optical element and the image plane is filled by an immersion medium with a refractive index substantially greater than 1. The position of the last optical element is adjustable in the direction of the optical axis. A positioning device is provided that positions at least the last optical element during immersion operation such that aberrations induced by disturbance are at least partially compensated.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 12/715,473, filed Mar. 2, 2010, which is a continuation of U.S. patent application Ser. No. 12/265,090, filed on Nov. 5, 2008, now U.S. Pat. No. 7,692,868, which is a continuation of U.S. patent application Ser. No. 11/955,662 filed on Dec. 13, 2007, now U.S. Pat. No. 7,463,423, which is a continuation of International Patent application PCT/EP2006/005059 filed on May 26, 2006, which claims to U.S. provisional application No. 60/690,544, filed Jun. 14, 2005.

BACKGROUND OF THE INVENTION

The invention relates to a lithography projection objective for imaging a pattern arranged in an object plane of the projection objective onto a substrate to be arranged in an image plane of the projection objective.

The invention further relates to a method for correcting image defects in the case of a lithography projection objective that can be tuned to immersion operation.

A projection objective of the type mentioned at the beginning is preferably used for microlithography projection exposure machines for producing semiconductor components and other finely structured subassemblies. A projection objective serves the purpose of projecting patterns from photomasks or graticules, which are also designated as masks or reticles, onto an object, that is to say a substrate, coated with a photosensitive layer, or onto a semiconductor wafer coated with photoresist, with very high resolution.

The resolution of the imaging of the pattern by the projection objective is proportional to the wavelength of the light used, and inversely proportional to the image-side numerical aperture of the projection objective. The resolution can therefore be improved with the aid of shorter wavelengths and higher numerical apertures. The numerical aperture NA is given by NA=n·sin Θ, n being the refractive index of the medium between the substrate and the last optical element of the projection objective.

Hitherto, use has predominantly been made of projection objectives in the case of which there exists in the image space between the exit surface of the last optical element of the projection objective and the image plane a finite working distance that is filled during operation with air or another suitable gas. Such systems are designated as “dry systems” or “dry objectives”. The working distance between the last optical element and the substrate is generally filled in this case with helium, nitrogen or another gas or a gas mixture with a refractive index n of approximately 1.

It follows from the previously mentioned relationship between the resolution and the image-side numerical aperture that the resolution can be raised when an immersion medium of high refractive index is introduced into the working distance between the exit surface of the last optical element and the substrate. This technique is designated as immersion lithography. A projection objective of this type is also designated as an “immersion system” or “immersion objective”. Some refractive projection objectives that are suitable for immersion lithography and have image-side numerical apertures NA>1 are disclosed in the patent applications DE 102 10 899 and PCT/EP 02/04846 of the same applicant.

A further advantage of an immersion objective consists in the possibility of obtaining a larger depth of field of the imaging in conjunction with the same numerical aperture as for a dry objective. This advantage is used in the projection objectives according to the invention.

In the case of an immersion objective, instead of being filled with a gas, the space between the exit surface of the last optical element of the projection objective and the substrate, which determines the working distance, is filled with an immersion medium of a refractive index substantially greater than 1. An immersion medium normally used at present is water, but it is possible, particularly within the scope of the present invention, to select other immersion media in accordance with needs and suitability.

Document EP 1 431 826 A2, which stems from the same applicant, describes how simple design means and manipulations can be used to tune a projection objective between a dry operation (dry configuration) and an immersion operation (immersion configuration). The projection objective described there has a multiplicity of optical elements that are arranged along an optical axis of the projection objective, the optical elements comprising a first group, following the object plane, of optical elements and a last optical element that follows the first group, is next to the image plane and defines an exit surface of the projection objective that is arranged at a working distance from the image plane. The last optical element is substantially free from refractive power and has no sag or only a slight one. The tuning method described there comprises varying the thickness of the last optical element, changing the refractive index of the space between the exit surface of the last optical element and the substrate by introducing or removing an immersion medium, and moreover preferably an axial displacement of the last optical element for the purpose of setting a suitable working distance in the dry operation of the projection objective. Moreover, it is provided to refine the tuning to the dry configuration or the immersion configuration by changing the air spaces between individual optical elements of the first group or by providing or varying aspheres.

The projection objective of the present invention can likewise be tuned between a dry configuration and an immersion configuration.

However, the present invention is based on a further aspect of such a projection objective that can be tuned between the dry configuration and the immersion configuration.

A temperature change usually occurs during operation of a projection objective. This can be global, homogenous or else local. For example, the air around the projection objective, the projection objective housing, the individual mounts of the optical elements, the optical elements themselves and the air or the gas inside the projection objective and, during immersion operation, the immersion liquid can heat up.

It has emerged that temperature changes have a different effect with regard to spherical image defects on a projection objective during immersion operation than on a projection objective in the dry configuration. In other words, dry objectives and immersion objectives differ from one another with regard to their sensitivity to temperature changes.

In the case of a projection objective in dry configuration, such spherical aberrations induced by temperature changes can be at least largely compensated even in the relatively high order by simply refocusing in which only the position of the substrate is adjusted in the direction of the optical axis. Specifically, a change in the working distance between the exit surface of the last optical element and the substrate leads in the case of a projection objective in dry configuration to very similar aberrations such as heating up of the projection objective, and so the aberrations induced by the heating up can be at least largely compensated by an appropriately directed displacement of the substrate, usually in conjunction with heating up, in a direction away from the last optical element.

It came out that this mode of procedure, specifically a correction of image defects on the basis of temperature changes solely by adjusting the position of the substrate does not lead in the case of a projection objective in immersion configuration to the result as in the case of a projection objective in dry configuration, that is to say in the case of such a focusing correction in which the Zernike coefficient Z4 is compensated to zero, higher spherical Zernike coefficients Z9, Z16, Z25, . . . remain and impair the imaging properties of the projection objective in immersion configuration.

SUMMARY OF THE INVENTION

It is the object of the invention to improve a projection objective that can be or is tuned to immersion operation with regard to its imaging properties or with regard to the correctability of image defects that are caused by a disturbance during immersion operation, such as a change in temperature, for example.

It is also the object of the invention to specify a method for correcting image defects of a projection objective that can be or is tuned to immersion operation, which can be carried out with the aid of simple means.

According to the invention, a lithography projection objective according to claim 1 is provided for achieving the first mentioned object.

According to the invention, a method for correcting aberrations in the case of a projection objective that can be, or is, tuned to immersion operation is specified according to claim 29 for the purpose of achieving the object mentioned in the second instance.

A lithography projection objective according to the invention for imaging a pattern to be arranged in an object plane of the projection objective onto a substrate to be arranged in an image plane of the projection objective has a multiplicity of optical elements that are arranged along an optical axis of the projection objective. The optical elements comprise a first group, following the object plane, of optical elements, and a last optical element, which follows the first group and is next to the image plane and which defines an exit surface of the projection objective and is arranged at a working distance from the image plane. The projection objective can be or is tuned with respect to aberrations for the case that the volume between the last optical element and the image plane is filled by an immersion medium with a refractive index substantially greater than 1. The position of the last optical element can be adjusted in the direction of the optical axis. A positioning device is provided that positions at least the last optical element during immersion operation such that aberrations induced by a disturbance caused by the operation of the projection objective are at least partially compensated.

The method according to the invention for correcting image defects in the case of a lithography projection objective that can be, or is, tuned to immersion operation comprises the step, in the event of a disturbance arising during immersion operation of the projection objective, of positioning at least the last optical element such that aberrations induced by the disturbance are at least partially compensated.

A disturbance in the case of the abovementioned projection objective or the abovementioned method is, for example, a change in temperature. The present invention is based on the finding that in the event of a change in temperature owing to heating up of the projection objective the working distance between the exit surface of the last optical element and the substrate is varied by the thermal expansion of the projection objective. However, since the immersion medium is located between the last optical element and the substrate during immersion operation, this change in the working distance leads to other sensitivities of the projection objective during immersion operation than by comparison with dry operation. In dry operation of the projection objective, the change in working distance has no influence on the aberrations, while the changed working distance, and thus the changed layer thickness of the immersion liquid during immersion operation, induces additional aberrations. These additional aberrations during immersion operation cannot be compensated solely by displacing the substrate in the direction of the optical axis, as in the case of the dry objective.

An instance of “disturbance” in the meaning of the present invention can also be one that is not caused by temperature, but is based, for example, on bubble formation in the immersion liquid, unevenness of the wafer surface, a locally differing wafer thickness or other geometry errors, and which renders refocusing necessary. This refocusing can be accomplished by displacing the wafer stage and/or displacing optical elements in the direction of the optical axis.

The inventive solution to this problem now consists, in the event of a disturbance, in positioning at least also the last optical element such that aberrations induced by the disturbance are at least partially compensated via the positioning of the last optical element. In particular, and preferably, by means of positioning of the last optical element the volume filled with the immersion medium, which can change in the event of disturbance such as a thermal expansion, can be set by means of the positioning device such that the aberrations induced by the disturbance, in particular aberrations of higher order, are at least largely compensated. By contrast with refocusing solely by adjusting the position of the substrate in the direction of the optical axis in the case of a projection objective during dry operation, now at least also the position of the last optical element is adjusted in the direction of the optical axis in order to keep the working distance, and thus the layer thickness of the immersion medium, preferably at a nominal value, while the gas-filled or air-filled space between the last optical element and the penultimate optical element of the projection objective is kept variable.

It is preferably also possible to adjust the position of the substrate in the direction of the optical axis, and the positioning device adjusts the last optical element in a ratio correlated with the adjustment of the position of the substrate. In particular, the ratio between the adjustment of the position of the substrate and the adjustment of the position of the last optical element can be selected to be 1:1.

With this type of “alternative focusing” in the immersion system by comparison with focussing in the dry system, the same focussing sensitivities are achieved in the immersion objective as in the case of focusing in the dry objective, in which only the substrate is displaced without adjusting the position of the last optical element in the direction of the optical axis.

Depending on requirement and degree of optimization, the ratio between the adjustment of the position of the substrate and the adjustment of the position of the last optical element can also be selected to be greater than or less than 1:1.

For the purpose of further optimizing the imaging properties of the projection objective during immersion operation, it is preferred in a first step to adjust only the position of the last optical element in the direction of the optical axis in order to restore a predetermined desired working distance, and in a second step the position of the last optical element and the position of the substrate are adjusted in the direction of the optical axis, preferably in the ratio of 1:1.

It is also possible here to interchange the first step and the second step in the sequence, or it can be provided to carry out these two steps in an interlocking fashion.

It has emerged that optimum corrections of the imaging properties, in particular even in higher orders of the spherical aberrations, that correspond to the achievable corrections in the dry configuration can be achieved by controlling the working distance filled with immersion medium to a nominal value that corresponds, for example, to the optimum value which is pre-calculated in the cold state of the projection objective, and the refocusing, as mentioned above, by simultaneously adjusting the position of the substrate and of the last optical element in the ratio of preferably 1:1.

The working distance is preferably measured before and/or during operation of the projection objective, in order to enable permanent control of the working distance. The respective measurement results are then used to adjust the position of the last optical element and/or of the substrate.

The measuring device preferably cooperates with an actuator in order to regulate the working distance to a nominal value. This enables unchanging optimum imaging properties of the projection objective during operation of the projection objective, that is to say the projection objective is capable of reacting to any sort of disturbance, for example changes in temperature, without manipulations from outside.

In a preferred alternative, it is also possible to proceed such that the position of the last optical element is adjusted so that aberrations can subsequently be at least largely compensated solely by adjusting the position of the substrate.

In this mode of procedure, the temperature sensitivities of the projection objective are therefore adapted during immersion operation such that, as in dry operation, they are again compatible with the sensitivities that exist from solely adjusting the substrate.

In the case of the projection objective, the last optical element is preferably assigned at least one actuator for adjusting the position of the last optical element in the direction of the optical axis.

Alternatively or cumulatively, the positioning device can have a mount for the last optical element that, upon heating up, moves the last optical element in a direction running counter to the thermal expansion of the projection objective.

This type of mounting technique for the last optical element with the use of materials with various coefficients of thermal expansion advantageously ensures that despite thermal expansion of the projection objective the working distance between the last optical element and the substrate can be kept at least approximately at the nominal value. This renders it possible even without additional actuators to compensate the aberrations induced by disturbance, or these are already avoided from the beginning.

In accordance with further preferred measures specified in the claims, the projection objective according to the invention can be tuned between dry operation and immersion operation.

Further advantages and features emerge from the following description and the attached drawing.

It is self-evident that the abovementioned features which are still to be explained below can be used not only in the combination respectively specified, but also in other combinations or on their own without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawing and explained in yet more detail here with reference thereto. In the drawing:

FIG. 1 shows a schematic projection objective in immersion configuration;

FIG. 2 shows the projection objective in FIG. 1 in dry configuration;

FIG. 3 shows a bar diagram for illustrating the influence of a displacement of the substrate in the direction of the optical axis on a wavefront change in a comparison between the projection objective in FIG. 2 (dry configuration) and the projection objective in FIG. 1 (immersion configuration);

FIG. 4 shows a bar diagram in which the fractions of higher spherical aberrations (Z9, Z16, Z25, Z36) in relation to the Z4 fraction of the spherical aberration are illustrated by comparison with the projection objective in FIG. 2 and the projection objective in FIG. 1;

FIG. 5 shows a bar diagram that shows the temperature sensitivities of the projection objective in FIG. 2 in comparison with the temperature sensitivities of the projection objective in FIG. 1;

FIG. 6 shows a bar diagram that illustrates the temperature sensitivities of the projection objective in FIG. 2 in comparison with those of the projection objective in FIG. 1 with and without correction of the Z4 fraction of the spherical aberration to the value zero;

FIGS. 7 a) and b) show a detail of the projection objective in FIG. 1 in two different states;

FIG. 8 shows a bar diagram similar to FIG. 6 although aberrations of higher order are illustrated after an identical displacement of the last optical element and the substrate for the projection objective in FIG. 1 in accordance with FIG. 7 b) (the ratio between the adjustment of the position of the substrate and the adjustment of the position of the last optical element being equal to 1:1);

FIG. 9 shows a bar diagram, comparable to the bar diagram in FIG. 8, after a further error correction of the projection objective in FIG. 1 (the ratio between the adjustment of the position of the substrate and the adjustment of the position of the last optical element not being equal to 1:1);

FIG. 10 shows a schematic of a mount for the last optical element that compensates or overcompensates a temperature-induced change in the working distance;

FIG. 11 shows an embodiment of a projection objective in immersion configuration, in which the present invention can be used; and

FIG. 12 shows a further embodiment of a projection objective in immersion configuration in which the present invention can likewise be implemented.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a projection objective, provided with the general reference numeral 10, in immersion configuration.

The projection objective 10 is used for the microlithographic production of semiconductor components and other finely structured subassemblies. The projection objective operated with ultraviolet light from the deep UV region (for example 193 nm) serves to image onto a substrate 14, which is arranged in an image plane 15 of the projection objective 10, a pattern 12 of a photomask (reticle) that is arranged in an object plane 13 of the projection objective.

The projection objective 10 has a multiplicity of optical elements in the form of lenses, but can also have both lenses and mirrors.

The optical elements are arranged along an optical axis 16 of the projection objective 10.

The optical elements comprise a first group 18 of optical elements that follow the object plane 13 or the pattern 12. Three optical elements 18 a, 18 b and 18 c of the first group 18 are illustrated by way of example in FIG. 1.

The optical elements further comprise a last optical element 20 that follows the first group 18 and is closest to the substrate 14 or the image plane 15.

The last optical element 20 is illustrated in FIG. 1 as a plane-parallel plate. However, the last optical element 20 can also respectively have on the entrance side and exit side a radius of curvature that are, however, only so large that aspheric aberrations induced by the displacement of the optical element 20 in the direction of the optical axis 16 are sufficiently small.

Like the optical elements of the first group, the last optical element 20 can be made from synthetic quartz glass. Furthermore, the last optical element 20 can also comprise a number of components in the direction of the optical axis.

An exit surface 22, facing the image plane 15, of the last optical element 20 also simultaneously forms the exit surface of the projection objective 10.

A distance between the exit surface 22 of the last optical element 20 and the surface of the substrate 14 defines a working distance 24.

The last optical element 20 is spaced sufficiently far away, for example, by a few millimeters, from the first group 18 of optical elements, such that the position of the last optical element 20 can be adequately adjusted in the direction of the optical axis 16.

The working distance 24 between the last optical element 20 and the substrate 14 is filled with an immersion medium 26. The immersion medium 26 is, for example, an immersion liquid, for example water, that has a refractive index of n≈1.437 given an operating wavelength of λ=193 nm.

In the configuration illustrated in FIG. 1, the projection objective 10 is designed for immersion operation, that is to say, with reference to the aberrations produced, it is designed for, that is to say tuned to, the presence of the immersion medium 26 at the working distance 24.

FIG. 2 shows the projection objective 10 in FIG. 1 in its dry configuration. The transition of the projection objective 10 in FIG. 1 (immersion configuration) to its dry configuration in FIG. 2 will firstly be described.

Starting with FIG. 1, the projection objective 10 is transformed as follows into its dry configuration. The immersion medium 26 is removed from the image space in a first step. This measure does not change the correction state before entry into the terminal element or into the last optical element 20 and at the exit of the terminal element 20. However, by removing the highly refractive, plane-parallel layer made from the immersion medium 26 from the image space, the overcorrecting effect thereof is taken away such that the correaction state in the image plane 15 corresponds to the undercorrected correction state at the exit surface 22.

In a further step, the thickness of the last optical element 20 is enlarged. In this case, the overcorrecting effect thereof increases with the rising thickness. In accordance with the refractive index of the plate material, the thickness is selected to be so wide that the overcorrection effected by the thicker plate, which now forms the last optical element 20, largely compensates the undercorrected correction state at the entrance to the last optical element 20.

In two further steps, a larger working distance 24 by comparison with the immersion configuration in FIG. 1 is set by axially displacing the last optical element 20 in the direction of the first group 18. This can be performed by axially displacing the last optical element 20 with the aid of a z-manipulator that can be driven electrically or in another way. It is also possible to mount the last optical element 20 individually and to use spacers to set a suitable spacing between the mounts of the first group 18 and the mount of the last optical element 20 by removing and/or installing spacers. Since the plane-parallel plate 20 is as free from refractive power as possible and does not sag, this axial displacement can be carried out without this having a measurable influence on the aberrations of the projection objective 10.

Furthermore, at least one of the lenses, for example the negative lens 18 b, in the first group 18 is mounted such that it can be displaced axially with the aid of a z-manipulator. A residual error can be compensated in this case by slightly displacing the lens 18 b in the direction of the object plane such that the completely tuned projection objective 10 in dry configuration has a sufficiently good correction state at the light exit.

The last-mentioned step, specifically the fine tuning with the aid of at least one manipulatable or variable optical element of the first group 18 can frequently be required in order to be able to meet tight specifications. In the event of lesser requirements, the first-mentioned steps (changing the refractive index in the image space by introducing or removing an immersion medium, varying the thickness of the last element 20, and displacing the last element 20 in order to change the working distance 24) can suffice in order to achieve a reconfiguration between immersion configuration and dry configuration (or vice versa).

It is described below how a disturbance or interference in the form of a temperature change affects the projection objective 10 in dry configuration (FIG. 2), and how the same disturbance affects the projection objective 10 in immersion configuration (FIG. 1), and how aberrations induced by the disturbance can be corrected.

Firstly, the fact is that the response of the projection objective 10 to an identical disturbance in the dry configuration and in the immersion configuration is virtually identical if, firstly, the presence of the immersion medium 26 is discounted. Such disturbance is encountered with the projection objective 10 in the dry configuration by adjusting the position of the substrate 14 in the direction of the optical axis 16 in order thereby to carry out a focus correction such that the Zernike coefficient Z4 vanishes in the middle of the field.

A displacement of the substrate 14 in the direction of the optical axis 16 by the amount ΔZ (compare FIGS. 1 and 2) leads in both systems to a wavefront change OPD for which it holds that:

OPD _(Δz)(ρ)=Δz·n√{square root over (1−(NA/n)²π²)}.  (1)

Here, n is the refractive index at the working distance 24, that is to say n≈1.000 for air in the dry configuration, or n≈1.437 for water in the immersion configuration for a given wavelength λ=193 nm. ρ is the normalized radial pupil coordinate.

The wavefront change OPD Δz in accordance with equation (1) can be developed in the customary way using Zernike polynomials:

OPD _(Δz)(ρ)=Δz·(f ₄(NA,n)·Z4(ρ)+f ₉(NA,n)·Z9(ρ)+f ₁₆(NA,n)·Z16(ρ)+f ₂₅(NA,n)·Z25(ρ)+f ₃₆(NA,n)·Z36(ρ))  (2)

The following Zernike coefficients Δz·f_(i)(NA, n) with NA′=NA/n are then yielded analytically in this expansion:

$\begin{matrix} {\mspace{79mu} {{f_{4}\left( {{NA},n} \right)} = {n \cdot \frac{4 - {NA}^{\prime 2} - {\sqrt{1 - {NA}^{\prime 2}}\left( {4 - {3{NA}^{\prime 2}} - {NA}^{\prime 4}} \right)}}{{5/2}{NA}^{\prime 4}}}}} & (3) \\ {\mspace{79mu} {{f_{9}\left( {{NA},n} \right)} = {n \cdot \frac{\begin{matrix} {48 - {84{NA}^{\prime 2}} + {35{NA}^{\prime 4}} -} \\ {\sqrt{1 - {NA}^{\prime 2}}\left( {48 - {60{NA}^{\prime 2}} + {11{NA}^{\prime 4}} + {NA}^{\prime 6}} \right)} \end{matrix}}{{21/2}{NA}^{\prime 6}}}}} & (4) \\ {\mspace{79mu} {{f_{16}\left( {{NA},n} \right)} = {n \cdot \frac{\begin{matrix} {320 - {740{NA}^{\prime 2}} + {504{NA}^{\prime 4}} -} \\ {\sqrt{1 - {NA}^{\prime 2}}\begin{pmatrix} {320 - {560{NA}^{\prime 2}} +} \\ {{264{NA}^{\prime 4}} - {23{NA}^{\prime 6}} - {NA}^{\prime 8}} \end{pmatrix}} \end{matrix}}{{45/2}{NA}^{\prime 8}}}}} & (5) \\ {{f_{25}\left( {{NA},n} \right)} = {{n \cdot \frac{\begin{matrix} {1792 - {4928{NA}^{\prime 2}} + {4752{NA}^{\prime 4}} -} \\ {{1848{NA}^{\prime 6}} + {231{NA}^{\prime 8}}} \end{matrix}}{{77/2}{NA}^{\prime 10}}} - {{n \cdot \sqrt{1 - {NA}^{\prime 2}}}\frac{\begin{pmatrix} {1792 - {4032{NA}^{\prime 2}} +} \\ {{2960{NA}^{\prime 4}} - {760{NA}^{\prime 6}} + {39{NA}^{\prime 8}} + {NA}^{\prime 10}} \end{pmatrix}}{{77/2}{NA}^{\prime 10}}}}} & (6) \\ {{f_{36}\left( {{NA},n} \right)} = {{n \cdot \frac{\begin{matrix} {9216 - {29952{NA}^{\prime 2}} + {36608{NA}^{\prime 4}} -} \\ {{20592{NA}^{\prime 6}} + {5148{NA}^{\prime 8}} - {429{NA}^{\prime 10}}} \end{matrix}}{{117/2}{NA}^{\prime 12}}} - {{n \cdot \sqrt{1 - {NA}^{\prime 2}}}\frac{\begin{pmatrix} {9216 - {25344{NA}^{\prime 2}} +} \\ {{25088{NA}^{\prime 4}} - {10640{NA}^{\prime 6}} + {1740{NA}^{\prime 8}} - {59{NA}^{\prime 10}} - {NA}^{\prime 12}} \end{pmatrix}}{{117/2}{NA}^{\prime 12}}}}} & (7) \end{matrix}$

It is to be seen from equations (3) to (7) that the Zernike coefficients Δz·f_(i), that is to say the induced spherical aberrations both of order Z4 and of higher orders Z9, Z16, Z25, Z36 are a function both of the numerical aperture and, in particular, of the refractive index n in the working distance 24.

FIG. 3 illustrates this state of affairs with reference to a numerical example. The bar diagram illustrates the wavefront changes, coded according to the Zernike coefficients Z4, Z9, Z16, Z25 and Z36 for a displacement of the substrate 14 by Δz=1 μm given a numerical aperture NA=0.93 for the projection objective in dry configuration (FIG. 2) and for the projection objective 10 in immersion configuration (FIG. 1). Of each pair of bars, the left-hand bar relates to the dry configuration, and the right-hand bar to the immersion configuration.

FIG. 4 illustrates the relative wavefront changes in the orders Z9, Z16, Z25 and Z36 referred to Z4. It follows in particular from FIG. 4 that a displacement of the substrate 14 in the direction of the optical axis 16 by the amount Δz has a lesser effect on the wavefront changes in the higher orders Z9, Z16, Z25, Z36 with the projection objective 10 in immersion configuration than in the dry configuration. That is to say, the sensitivity of the projection objective 10 to the z-displacement of the substrate 14 in the higher Zernike coefficients is less in immersion configuration than in the dry configuration. The consequence of this is that the method of focus correction by displacing the substrate 14 in the direction of the optical axis 16 that is applied in the dry configuration of the projection objective 10 has less influence on the wavefront changes or aberrations of higher order.

While previously the different focus sensitivities of the projection objective 10 in the dry configuration have been considered by comparison with the immersion configuration, in the following the sensitivity of the projection objective 10 in both configurations is explained with regard to a disturbance in the form of a temperature change.

In a simulation of aberrations induced by a global, homogeneous temperature change, for example in the air (or another gas) around the projection objective 10, in the projection objective housing with the individual mounts, in the gas inside the projection objective 10, in the lenses and in the immersion liquid 24, the sensitivities of the following effects were considered:

-   1. Change in the lens geometries—that is to say the thicknesses and     radii—through the thermal expansion of the lens material; -   2. Changes in spacings through the thermal expansion of the     projection objective housing (metal mounts):     -   a. between the lenses (“air spaces”),     -   b. between the pattern 12 (reticle) and a first optical element         of the first group 18 of optical elements,     -   c. between the last optical element 20 and the substrate 14; -   3. Changes in refractive index Δn=dn/dT ΔT of the lens material     (quartz, CaF₂); -   4. Changes in refractive index Δn=dn/dT ΔT     -   a. of the gas between the individual optical elements of the         first group 18,     -   b. of the air (or the other gas) between the pattern 12 and the         first optical element of the group 18,     -   c. in the immersion liquid 24 between the last optical element         20 and the substrate 14 in the immersion system or in the air         (or the other gas) in the case of the dry system.

FIG. 5 shows the temperature sensitivities without focus correction, that is to say without displacement of the substrate 14 in the direction of the optical axis with reference to the spherical Zernike coefficients in the center of the field in a comparison between the projection objective 10 in the dry configuration and the projection objective 10 in the immersion configuration, once again the left-hand bar of each pair of bars referring to the dry configuration, and the right-hand bar referring to the immersion configuration.

It emerges from FIG. 5 that the dry configuration and the immersion configuration of the projection objective 10 differ considerably from one another with regard to the temperature sensitivities, at least in the orders Z4 and Z9. These differences between the dry configuration and the immersion configuration result from the above-mentioned contributions 2.c and 4.c to the temperature effects, that is to say the differences are a consequence of the presence of the immersion liquid 26 at the working distance 24 between the last optical element 20 and the substrate 14. Of the two contributions 2.c and 4.c, the contribution 2.c, that is to say the change in the working distance 24, is the dominating additional contribution to the aberrations in the immersion configuration. This can be explained in that the temperature-induced thermal expansion of the projection objective 10 displaces the last optical element 20 in the direction of the substrate 14. The working distance 24 is consequently reduced. Whereas this has no influence on the aberrations in the dry configuration, the change in the working distance 24 in the immersion configuration induces a changed layer thickness of the immersion liquid 26 that induces additional aberrations. All other above-mentioned contributions to the temperature effects yield virtually identical sensitivities in the two systems.

FIG. 6 illustrates with the aid of a further bar diagram the extent to which the previously described aberrations induced by a temperature change can be compensated only by adjusting the position of the substrate 14 in the direction of the optical axis 16.

FIG. 6 shows the wavefront changes OPD/T, caused by temperature changes, for the dry configuration and for the immersion configuration, respectively without and with focus correction solely by displacing the substrate 14 in the direction of the optical axis, in a fashion split up with reference to the Zernike coefficients Z9, Z16, Z25, Z36 (Z4=0 after the focus correction).

Of the four bars relating to each of the coefficients Z9, Z16, Z25, Z36, the first bar relates to the dry configuration without focus correction, the second bar to the dry system with focus correction (Z4=0), the third bar to the immersion configuration without focus correction, and the fourth bar to the immersion configuration with focus correction (Z4=0) solely by displacing the substrate 14 in the direction of the optical axis.

It is clear from FIG. 6 that in the dry configuration the higher spherical sensitivities Z9, Z16, Z25, Z36 relating to a homogeneous temperature change have a similar ratio to the Z4 fraction as the focus sensitivities in the case of adjusting the position of the substrate 14 in the direction of the optical axis 16. As a result of this circumstance, a focus correction, that is to say a correction such that Z4=0 in the middle of the field, solely by adjusting the position of the substrate 14 in the direction of the optical axis simultaneously also adequately corrects substantial contributions of the higher spherical aberrations Z9, Z16, Z25, Z36 in the dry configuration. By contrast, in the immersion configuration the crosstalk in the higher spherical Zernike coefficients is substantially smaller in the case of Z4 correction solely by adjusting the position of the substrate 14, as has been explained above with reference to FIGS. 3 and 4. The consequence of this is that in the case of a complete compensation of Z4 in the immersion configuration it is still only small fractions of the higher Zernike coefficients Z9, Z16, Z25, Z36 that are also compensated, and therefore large fractions of these aberrations of higher order remain as residual errors. Consequently, it is not sufficient to correct aberrations simply by adjusting the position of the substrate 14 in the direction of the optical axis, that is to say nothing but a focus correction to Z4=0 in the immersion configuration. This means that in the immersion configuration as contrasted with the dry configuration for the case of an identical temperature change there remains a residual error Z9 that is larger by a factor of approximately 7 and residual errors that are approximately four to five times larger for the other spherical Zernike coefficients of higher order when only one focus correction is performed by adjusting the position of the substrate 14.

With reference to FIGS. 7 a) and b), it is described below how the residual errors of the projection objective 10 can be reduced in immersion configuration by means of an alternative type of focusing.

FIG. 7 a) shows the projection objective 10 in accordance with FIG. 1 in the region of the last optical element 20 and of the penultimate optical element 18 c that forms the last optical element of the first group 18 of optical elements of the projection objective 10. The space between the penultimate optical element 18 c and the last optical element 20 is filled with a gas having a refractive index n of approximately 1.

In accordance with FIG. 7 b), the projection objective 10 has a positioning device 28 that comprises an actuator 30 and a measuring device 32. The actuator 30 is capable of positioning the last optical element 20 in the direction of the optical axis 16 (z direction) as is indicated by an arrow 30 a. The actuator 30 is further capable of likewise positioning the substrate 14 in the direction of the optical axis 16, as indicated by an arrow 30 b.

The actuator 30 is capable, in particular, of adjusting the position of the last optical element 20 and of the substrate 14 in a mutually correlated ratio in the direction of the optical axis 16.

The aim firstly is to discuss what is the result of a common adjustment of the position of the last optical element 20 and of the substrate 14 in the same direction in a ratio of 1:1 as is illustrated in FIG. 7 b) by comparison with FIG. 7 a).

Adjusting the position of the last optical element 20 enlarges the air space 34 between the penultimate optical element 18 c and the last optical element 20 by the amount Δz_(LR) when the optical element 20 is displaced by the amount Δz (just like the substrate 14).

The wavefront change OPD_(Δz,LR) owing to the enlargement of the air space 34 is then given by

OPD _(Δz,LR)(ρ)=Δz·n′√{square root over (1−(NA/n′)²ρ²)}  (8)

Here, n′ is the refractive index of the gas in the air space 34 upstream of the last optical element 20. Comparing equation (8) with equation (1) shows that this type of focusing in the projection objective 10 in immersion configuration leads to the same change in the wavefront as does a corresponding sole displacement of the substrate 14 in the dry configuration, since the refractive index n′˜1 in the last air space 34 upstream of the last optical element 20 is virtually identical to the refractive index n˜1 of the air in the dry system. Consequently, the projection objective 10 in the dry configuration and in the immersion configuration then have the same focus sensitivities (equations (2) to (7)) with the same crosstalk to the higher spherical Zernike coefficients Z9, Z16, Z25, Z36.

FIG. 8 shows a similar illustration to that in FIG. 6, the fourth bar in relation to each of the Zernike coefficients Z9, Z16, Z25, Z36 showing the residual aberrations for the projection objective 10 in immersion configuration after an identical displacement of the last optical element 20 and of the substrate 14.

Comparing this respective fourth bar with the respective fourth bar in FIG. 6 shows that the residual aberrations in the higher Zernike coefficients Z9, Z16, Z25, Z36 are substantially reduced, and differ from the residual aberrations in the dry configuration only by factors of approximately 1.7 to 2.7.

It is described below how the residual aberrations of the projection objective 10 in the immersion configuration can be yet further reduced.

A further reduction in the residual aberrations of the projection objective 10 in immersion configuration is achieved by setting the working distance 24 between the last optical element 20 and the substrate 14 solely by adjusting the position of the last optical element 20 to a nominal value (nominal working distance), something which can likewise be carried out with the aid of the actuator 30. The nominal value can in this case be the originally set optimum working distance in immersion configuration if no disturbance such as a temperature-induced expansion of the system is present.

The wavefront change OPD_(Δz,LLE) owing to displacement of the last optical element 20 by the path Δz in the direction of the optical axis is then yielded as the difference between the wavefront change OPD_(Δz,LR) by enlarging the last air space 34 (FIG. 7 b)) and the wavefront change OPD_(Δz,S) by adjusting the position of the substrate 14 in the direction of the optical axis 16:

OPD _(Δz,LLE)(ρ)=OPD _(Δz,LR)(ρ)−OPD _(Δz,S)(ρ)=Δz[n′√{square root over (1−(NA/n′)²ρ²)}−n√{square root over (1−(NA/n)²ρ²)}]  (9)

Here, n is the refractive index of the immersion medium 26, and n′ is the refractive index of the gas in the air space 34 upstream of the last optical element 20.

The (sole) adjustment of the position of the last optical element 20 can now be used to fully compensate again the displacement of the last optical element 20 in the direction of the substrate 14 induced by the thermal expansion of the projection objective 10.

The result of this mode of procedure is illustrated in FIG. 9.

FIG. 9 shows for the first bar (seen from the left) relating to each Zernike coefficient Z9, Z16, Z25, Z36 the aberrations of a projection objective 10 in dry configuration due to a disturbance in the form of a temperature change, no focus correction yet having been performed. (The Zernike coefficient Z4, which is not shown in this figure, does not vanish here.)

The respective second bar in FIG. 9 shows the residual aberrations of the projection objective 10 in dry configuration after a focus correction solely by displacing the substrate 14 in the direction of the optical axis 16. (The Zernike coefficient Z4, which is not shown in this figure, vanishes here.)

The respective third bar in FIG. 9 shows in relation to each Zernike coefficient Z9, Z16, Z25, Z36 the aberrations of the projection objective 10 in immersion configuration due to a disturbance in the form of a temperature change without focus correction, the fourth bar shows the aberrations after setting the working distance 24 between the last optical element 20 and the substrate 14 to a desired working distance that corresponds, or can correspond, to the originally set working distance before commissioning of the projection objective 10, and the fifth bars show the residual aberrations after additional common adjustment of the position of the last optical element 20 and of the substrate 14 in the direction of the optical axis in the ratio of 1:1.

Comparing the first and fourth bars relating to each Zernike coefficient in FIG. 9 reveals that restoring the desired working distance between the last optical element 20 and the substrate 14 yields sensitivities that are virtually identical to the not refocused sensitivities of the projection objective 10 in dry configuration.

Comparing the second and fifth bars in relation to each Zernike coefficient in FIG. 9 reveals that these show identical residual aberrations for the projection objective 10 in dry configuration after focus correction, and identical residual aberrations for the projection objective 10 in immersion configuration after adjusting the position of the last optical element 20 in order to set a desired working distance, and identical adjustment of the position of the last optical element 20 and of the substrate 14 in the direction of the optical axis 16.

The focus correction (Z4=0) is carried out by correlated adjustment of the position of the last optical element 20 and the substrate 14. This now results in the same corrective action as in the case of the projection objective 10 in dry configuration (identical focus sensitivities), and virtually identical and sufficiently small residual errors of the higher spherical aberrations are achieved.

During operation of the projection objective 10 in immersion configuration, the working distance 24 can be controlled by means of the measuring device 32, and it is then possible on the basis of the respective measurement results to use the actuator 30 to keep the working distance 24 at the desired working distance, in the manner of a control loop.

FIG. 10 illustrates diagrammatically an embodiment with the aid of which, on the basis of a specific mounting technique for the last optical element 20, it is already possible to keep the working distance 24 between the last optical element 20 and the substrate 14 with reference to temperature changes at the set point, or to position the last optical element 20 for the purpose of minimizing aberrations.

The last optical element 20 is held in a mount 20 a that is connected to a mount 19 of an optical element of the first group 18 of optical elements of the projection objective 10 at a point 21. The mount 20 a has, in particular, a thermal expansion coefficient that is large by comparison with the thermal expansion coefficient of the mount 19.

If, by heating up, the mount 19 now expands in the direction of an arrow 23, this would reduce the working distance 24. However, owing to the heating up the mount 20 a also expands, but in the opposite sense to the expansion of the mount 19 in accordance with an arrow 25, the result being not to diminish the working distance 24 but to keep it substantially constant. It is thereby possible to keep the working distance 24 at the nominal value.

However, it is also possible to provide not to keep the working distance 24 at the nominal value by means of the previously described mounting technique, but to fashion the mount 20 a for the last optical element 20 such that it not only compensates the change in the working distance 24, but overcompensates it in such a way that the above-described customary focus correction, that is to say solely adjusting the position of the substrate 14, leads again to the same results for the correction of aberrations. Thus, with this mode of procedure the temperature sensitivities of the projection objective are adapted in terms of design in such a way that they are once again compatible with the focus sensitivities as in the dry configuration.

The following measures are provided with reference, again, to FIGS. 1 and 2, which show the projection objective 10 in immersion configuration and in dry configuration, respectively, in order to tune the projection objective 10 between the dry configuration and the immersion configuration.

A large distance that enables a substantial axial displacement of the last optical element 20 exists between the first group 18 and the last optical element 20.

The tunability between the immersion configuration in FIG. 1 and the dry configuration in FIG. 2 of the projection objective 10 is preferably achieved with the aid of a variation in the thickness of the last optical element 20, preferably in conjunction with a displacement of the last optical element 20 relative to the image plane 15, it being necessary, however, not to confuse this method with the previously described method for correcting aberrations of the projection objective 10 in the immersion configuration.

The last optical element 20 is, furthermore, exchangeable.

The last optical element 20 can have a variable thickness, the last optical element 20 preferably having a thickness that can be varied without removing material or adding material. This is preferably achieved by virtue of the fact that the last optical element 20 comprises a number of mutually detachable components that are arranged at a spacing from one another or are neutrally interconnected in optical terms, it being preferred for components of the last optical element 20 to consist of different optical materials, preferably at least one component consisting of fluoride crystal, in particular of lithium fluoride or calcium fluoride.

The optical material, adjacent to the exit surface 22, of the last optical element 20 preferably has a refractive index n_(E) that is close to the refractive index n_(I) of the immersion medium 26, it being preferred for a ratio n_(I)/n_(E) to be more than 0.8, in particular more than 0.9.

Furthermore, the first group 18 of optical elements also has at least one displaceable optical element, but preferably a number of, in particular at least five, displaceable optical elements, at least one of the displaceable optical elements being displaceable along the optical axis 16.

A free space upstream of the previously mentioned displaceable element and/or downstream of the displaceable element is in this case preferably dimensioned to be so large that displacing the at least one displaceable optical element renders it possible to correct at least a fraction of aberrations that result from adapting the last optical element 20 to the immersion medium 26. The projection objective 10 can be assigned at least one exchangeable optical correction element that preferably has at least one aspheric surface. Furthermore, at least one optical element of the first group 18 can have at least one optical surface with a surface curvature that can be varied reversibly or permanently.

The projection objective 10 is designed such that, when use is made of the immersion medium 26, that is to say in the immersion configuration, it has an image-side numerical aperture NA<1 between exit surface 22 and image plane 15, the image-side numerical aperture preferably being between approximately 0.7 and 1.0, in particular between 0.8 and 1.0.

It is further provided that the last optical element 20 can be removed from the projection objective 10 and be replaced by a plane-parallel plate that is large by comparison with the exit surface of the projection objective 10 and can be laid over a large area of the substrate 14 to be exposed.

FIGS. 11 and 12 demonstrate particular exemplary embodiments of projection objectives in the case of which the present invention can be implemented.

TABLE 1 j29o REFRACTIVE INDEX ½ FREE SURFACE RADII THICKNESSES LENSES 248.38 nm DIAMETER 0 0.000000000 32.000000000 1.00000000 54.410 1 0.000000000 10.587540450 L710 0.99998200 61.093 2 −2417.351767120 13.126300000 SUPRA1 1.50833811 63.132 3 −248.195466920 7.359264018 L710 0.99998200 63.945 4 −168.131361870 10.000000000 SUPRA1 1.50833811 64.202 5 328.986124739 7.907519166 L710 0.99998200 70.046 6 671.742152743 22.614900000 SUPRA1 1.50833811 71.945 7 −219.346865952 1.054978296 L710 0.99998200 73.402 8 351.854459479 21.378800000 SUPRA1 1.50833811 77.449 9 −417.329819985 0.748356148 L710 0.99998200 77.668 10 266.259242017 26.426700000 SUPRA1 1.50833811 76.971 11 −418.068287643 0.747164758 L710 0.99998200 75.964 12 195.049526899 10.000000000 SUPRA1 1.50833811 69.816 13 112.784218098 27.264697553 L710 0.99998200 64.221 14 −548.976305020 10.000000000 SUPRA1 1.50833811 63.660 15 167.581609987 25.042515270 L710 0.99998200 61.992 16 −203.629259785 10.000000000 SUPRA1 1.50833811 62.349 17 360.120642869 28.995838980 L710 0.99998200 66.965 18 −127.653905514 12.696400000 SUPRA1 1.50833811 68.153 19 −1103.725724970 17.018787360 L710 0.99998200 81.984 20 −225.898831342 23.521200000 SUPRA1 1.50833811 84.684 21 −171.063497139 1.574450554 L710 0.99998200 92.606 22 −22770.163604600 38.438000000 SUPRA1 1.50833811 109.997 23 −229.816390281 0.749282985 L710 0.99998200 113.270 24 1170.594630540 38.363100000 SUPRA1 1.50833811 123.579 25 −320.184892150 0.749629640 L710 0.99998200 124.514 26 335.012872058 39.596800000 SUPRA1 1.50833811 124.658 27 −764.462984962 2.214257730 L710 0.99998200 123.947 28 270.136227728 25.935800000 SUPRA1 1.50833811 112.963 29 1248.618077510 4.352014987 L710 0.99998200 110.825 30 177.098661261 18.578800000 SUPRA1 1.50833811 96.632 31 131.459110961 48.405871098 L710 0.99998200 84.997 32 −254.431714105 10.000000000 SUPRA1 1.50833811 83.694 33 149.734192113 49.515509852 L710 0.99998200 77.858 34 −137.204786283 10.000000000 SUPRA1 1.50833811 78.232 35 1410.223675540 43.391488727 L710 0.99998200 89.345 36 −134.825941720 35.292100000 SUPRA1 1.50833811 91.736 37 −168.418502871 3.480235112 L710 0.99998200 110.924 38 −350.805989269 24.010800000 SUPRA1 1.50833811 123.372 39 −244.301424027 6.015284795 L710 0.99998200 128.258 40 4941.534628580 43.549100000 SUPRA1 1.50833811 147.192 41 −357.889527255 2.367042190 L710 0.99998200 149.417 42 1857.663670230 40.932000000 SUPRA1 1.50833811 156.043 43 −507.091567715 −0.213252954 L710 0.99998200 156.763 44 0.000000000 0.962846248 L710 0.99998200 155.516 45 637.188120359 28.431900000 SUPRA1 1.50833811 156.869 46 −4285.746531360 0.749578310 L710 0.99998200 156.617 47 265.928249908 45.432900000 SUPRA1 1.50833811 152.353 48 1127.170329670 57.049328626 L710 0.99998200 150.272 49 −273.057181282 24.571800000 SUPRA1 1.50833811 149.389 50 −296.450446798 2.401860529 L710 0.99998200 150.065 51 −317.559071036 23.847600000 SUPRA1 1.50833811 148.110 52 −297.103672940 0.819938446 L710 0.99998200 148.158 53 223.869192775 28.117900000 SUPRA1 1.50833811 122.315 54 548.591751129 0.749776549 L710 0.99998200 120.110 55 123.937471688 34.861300000 SUPRA1 1.50833811 99.291 56 211.883788830 0.738299719 L710 0.99998200 93.879 57 121.391085072 21.109500000 SUPRA1 1.50833811 82.929 58 178.110541498 13.722409422 L710 0.99998200 77.266 59 314.102464129 10.000000000 SUPRA1 1.50833811 71.524 60 60.563892001 10.471596266 L710 0.99998200 49.697 61 71.706607533 10.069000000 SUPRA1 1.50833811 48.032 62 53.184242317 0.713865261 L710 0.99998200 40.889 63 48.728728866 24.194000000 SUPRA1 1.50833811 39.865 64 325.049018458 16.249640231 L710 0.99998200 35.979 65 0.000000000 3.000000000 SUPRA1 1.50833811 16.879 66 0.000000000 2.000000000 IMMERS 1.40000000 14.998 67 0.000000000 0.000000000 1.00000000 13.603

TABLE 2 j30o REFRACTIVE INDEX ½ FREE SURFACE RADII THICKNESSES LENSES 248.38 nm DIAMETER 0 0.000000000 32.000000000 1.00000000 54.410 1 0.000000000 10.283889256 L710 0.99998200 61.093 2 −2417.351767120 13.126300000 SUPRA1 1.50833811 63.069 3 −248.195466920 7.293007084 L710 0.99998200 63.884 4 −168.131361870 10.000000000 SUPRA1 1.50833811 64.137 5 328.986124739 8.273191790 L710 0.99998200 69.971 6 671.742152743 22.614900000 SUPRA1 1.50833811 72.045 7 −219.346865952 0.447882685 L710 0.99998200 73.489 8 351.854459479 21.378800000 SUPRA1 1.50833811 77.419 9 −417.329819985 0.643718463 L710 0.99998200 77.636 10 266.259242017 26.426700000 SUPRA1 1.50833811 76.935 11 −418.068287643 1.297611013 L710 0.99998200 75.923 12 195.049526899 10.000000000 SUPRA1 1.50833811 69.627 13 112.784218098 26.146948060 L710 0.99998200 64.049 14 −548.976305020 10.000000000 SUPRA1 1.50833811 63.646 15 167.581609987 26.480913850 L710 0.99998200 61.963 16 −203.629259785 10.000000000 SUPRA1 1.50833811 62.465 17 360.120642869 28.474843347 L710 0.99998200 67.077 18 −127.653905514 12.696400000 SUPRA1 1.50833811 68.070 19 −1103.725724970 17.347391549 L710 0.99998200 81.856 20 −225.898831342 23.521200000 SUPRA1 1.50833811 84.765 21 −171.063497139 1.525859924 L710 0.99998200 92.671 22 −22770.163604600 38.438000000 SUPRA1 1.50833811 110.016 23 −229.816390281 0.449372011 L710 0.99998200 113.280 24 1170.594630540 38.363100000 SUPRA1 1.50833811 123.463 25 −320.184892150 0.449220757 L710 0.99998200 124.404 26 335.012872058 39.596800000 SUPRA1 1.50833811 124.508 27 −764.462984962 0.448529485 L710 0.99998200 123.785 28 270.136227728 25.935800000 SUPRA1 1.50833811 113.275 29 1248.618077510 4.599063715 L710 0.99998200 111.173 30 177.098661261 18.578800000 SUPRA1 1.50833811 96.787 31 131.459110961 48.903368693 L710 0.99998200 85.123 32 −254.431714105 10.000000000 SUPRA1 1.50833811 83.644 33 149.734192113 49.544589669 L710 0.99998200 77.792 34 −137.204786283 10.000000000 SUPRA1 1.50833811 78.174 35 1410.223675540 43.113042129 L710 0.99998200 89.233 36 −134.825941720 35.292100000 SUPRA1 1.50833811 91.558 37 −168.418502871 4.049119334 L710 0.99998200 110.696 38 −350.805989269 24.010800000 SUPRA1 1.50833811 123.308 39 −244.301424027 5.341877309 L710 0.99998200 128.188 40 4941.534628580 43.549100000 SUPRA1 1.50833811 146.729 41 −357.889527255 4.028668923 L710 0.99998200 148.997 42 1857.663670230 40.932000000 SUPRA1 1.50833811 155.818 43 −507.091567715 −1.371361371 L710 0.99998200 156.540 44 0.000000000 2.120040201 L710 0.99998200 155.343 45 637.188120359 28.431900000 SUPRA1 1.50833811 156.764 46 −4285.746531360 0.447699567 L710 0.99998200 156.510 47 265.928249908 45.432900000 SUPRA1 1.50833811 152.266 48 1127.170329670 56.966580248 L710 0.99998200 150.172 49 −273.057181282 24.571800000 SUPRA1 1.50833811 149.291 50 −296.450446798 2.661459751 L710 0.99998200 149.961 51 −317.559071036 23.847600000 SUPRA1 1.50833811 147.915 52 −297.103672940 0.449161173 L710 0.99998200 147.956 53 223.869192775 28.117900000 SUPRA1 1.50833811 122.290 54 548.591751129 1.339172987 L710 0.99998200 120.081 55 123.937471688 34.861300000 SUPRA1 1.50833811 99.087 56 211.883788830 0.952940583 L710 0.99998200 93.588 57 121.391085072 21.109500000 SUPRA1 1.50833811 82.604 58 178.110541498 13.676325222 L710 0.99998200 76.860 59 314.102464129 10.000000000 SUPRA1 1.50833811 71.076 60 60.563892001 10.077651049 L710 0.99998200 49.477 61 71.706607533 10.069000000 SUPRA1 1.50833811 47.911 62 53.184242317 0.732248727 L710 0.99998200 40.780 63 48.728728866 24.194000000 SUPRA1 1.50833811 39.753 64 325.049018458 4.167687088 L710 0.99998200 35.772 65 0.000000000 5.000000000 SUPRA1 1.50833811 32.831 66 0.000000000 12.000000000 L710 0.99998200 29.694 67 0.000000000 0.000000000 1.00000000 13.603

TABLE 3 j31o REFRACTIVE INDEX ½ FREE SURFACE RADII THICKNESSES LENSES 157.63 nm DIAMETER  0 0.000000000 38.482288093 1.00000000 85.333  1 304.292982078 22.168809366 CAF2HL 1.55840983 92.476  2 2741.794481050 96.128678854 1.00000000 92.204  3 0.000000000 0.000000000 −1.00000000 131.930 REFL  4 0.000000000 −467.095641350 −1.00000000 90.070  5 199.893955036 −10.268444544 CAF2HL −1.55840983 91.280  6 486.702942680AS −26.734713685 −1.00000000 96.529  7 186.738998389 −10.064297945 CAF2HL −1.55840983 99.240  8 447.975139348 −19.001496621 −1.00000000 111.362  9 243.529966034 19.001496621 1.00000000 114.369 REFL 10 447.975139348 10.064297945 CAF2HL 1.55840983 112.384 11 186.738998389 26.734713685 1.00000000 102.903 12 486.702942680AS 10.268444544 CAF2HL 1.55840983 101.523 13 199.893955036 464.738613843 1.00000000 96.499 14 0.000000000 0.000000000 −1.00000000 115.398 REFL 15 0.000000000 −100.235657635 −1.00000000 92.746 16 −536.442986965 −25.379215206 CAF2HL −1.55840983 94.306 17 629.049380815 −7.436012624 −1.00000000 93.787 18 0.000000000 −118.304806660 −1.00000000 91.342 19 −312.177007433AS −24.720749191 CAF2HL −1.55840983 94.928 20 −734.696609024 −220.443381712 −1.00000000 94.168 21 −277.004238298AS −15.426909916 CAF2HL −1.55840983 96.206 22 −460.130899964 −73.782961291 −1.00000000 95.245 23 −158.318468619 −30.586960517 CAF2HL −1.55840983 91.460 24 −162.867000225 −41.632945268 −1.00000000 84.793 25 419.508310212 −20.539965049 CAF2HL −1.55840983 84.016 26 −238.581080262 −31.955227253 −1.00000000 85.006 27 −430.197019246 −30.182066783 CAF2HL −1.55840983 92.237 28 691.939037816AS −23.703096035 −1.00000000 93.527 29 −241.462660758AS −10.000000000 CAF2HL −1.55840983 97.681 30 −182.472613831 −25.656103361 −1.00000000 96.159 31 −420.041190250 −36.705938298 CAF2HL −1.55840983 98.541 32 324.867666879 −43.586137768 −1.00000000 99.096 33 −44866.873107000 36.893151865 −1.00000000 93.979 34 −149.830817441 −28.311419778 CAF2HL −1.55840983 94.246 35 −315.631878253AS −18.939811826 −1.00000000 91.369 36 −172.862510793 −12.271843841 CAF2HL −1.55840983 87.996 37 −115.635345524 −27.567353538 −1.00000000 81.847 38 −229.213645994AS −32.436472831 CAF2HL −1.55840983 82.617 39 474.721571790 −3.611495525 −1.00000000 81.971 40 −152.435372054 −30.802088433 CAF2HL −1.55840983 75.907 41 −530.778945822 −8.465514650 −1.00000000 70.966 42 −159.504999222 −41.060952888 CAF2HL −1.55840983 63.576 43 3040.455878600 −4.225976128 −1.00000000 51.729 44 −226.630329417AS −24.123224774 CAF2HL −1.55840983 44.179 45 897.778633917 −8.617797536 −1.00000000 33.827 46 0.000000000 −8.000000000 CAF2HL −1.55840983 22.352 47 0.000000000 −2.000000000 IMMERS −1.39000000 18.217 48 0.000000000 0.000000000 −1.00000000 17.067 ASPHERIC CONSTANTS SURFACE NO. 6 K 0.0000 C1 3.87858881e−009 C2 −1.57703627e−013 C3 1.62703226e−017 C4 −1.12332671e−021 C5 −1.51356191e−026 C6 8.57130323e−031 SURFACE NO. 12 K 0.0000 C1 3.87858881e−009 C2 −1.57703627e−013 C3 1.62703226e−017 C4 −1.12332671e−021 C5 −1.51356191e−026 C6 8.57130323e−031 SURFACE NO. 19 K 0.0000 C1 3.62918557e−009 C2 6.75596543e−014 C3 5.68408321e−019 C4 −6.78832654e−023 C5 6.78338885e−027 C6 −2.05303753e−031 SURFACE NO. 21 K 0.0000 C1 1.19759751e−008 C2 7.35438590e−014 C3 7.03292772e−019 C4 −1.26321026e−023 C5 −3.01047364e−027 C6 2.08735313e−031 SURFACE NO. 28 K 0.0000 C1 −8.39294529e−009 C2 −3.39607506e−013 C3 8.76320979e−018 C4 −1.43578199e−021 C5 5.59234999e−026 C6 2.01810948e−030 SURFACE NO. 29 K 0.0000 C1 1.74092829e−008 C2 −1.69607632e−013 C3 1.18281063e−017 C4 −3.08190938e−021 C5 1.70082968e−025 C6 −1.68479126e−030 SURFACE NO. 35 K 0.0000 C1 −2.14453018e−008 C2 6.73947641e−013 C3 −4.84677574e−017 C4 5.99264335e−021 C5 −2.87629386e−025 C6 3.90592520e−031 SURFACE NO. 38 K 0.0000 C1 1.60415031e−008 C2 4.78837509e−015 C3 2.08320399e−016 C4 −2.87713700e−020 C5 1.77485272e−024 C6 −1.93501550e−029 SURFACE NO. 44 K 0.0000 C1 −6.56394686e−008 C2 −8.25210588e−012 C3 −1.27328625e−016 C4 −1.16616292e−020 C5 −1.58133131e−023 C6 6.39526832e−027

TABLE 4 j32o REFRACTIVE INDEX ½ FREE SURFACE RADII THICKNESSES LENSES 157.63 nm DIAMETER  0 0.000000000 36.500665837 1.00000000 85.333  1 304.292982078 22.168809366 CAF2HL 1.55840983 92.166  2 2741.794481050 96.128678854 1.00000000 91.891  3 0.000000000 0.000000000 −1.00000000 131.415 REFL  4 0.000000000 −467.820384551 −1.00000000 89.765  5 199.893955036 −10.268444544 CAF2HL −1.55840983 91.269  6 486.702942680AS −26.059978075 −1.00000000 96.632  7 186.738998389 −10.064297945 CAF2HL −1.55840983 99.260  8 447.975139348 −19.256116633 −1.00000000 111.485  9 243.529966034 19.256116633 1.00000000 114.609 REFL 10 447.975139348 10.064297945 CAF2HL 1.55840983 112.551 11 186.738998389 26.059978075 1.00000000 103.039 12 486.702942680AS 10.268444544 CAF2HL 1.55840983 101.801 13 199.893955036 465.028501331 1.00000000 96.752 14 0.000000000 0.000000000 −1.00000000 115.771 REFL 15 0.000000000 −100.235657635 −1.00000000 93.044 16 −536.442986965 −25.379215206 CAF2HL −1.55840983 94.574 17 629.049380815 −8.746601911 −1.00000000 94.056 18 0.000000000 −116.715874811 −1.00000000 91.368 19 −312.177007433AS −24.720749191 CAF2HL −1.55840983 94.620 20 −734.696609024 −220.365529295 −1.00000000 93.861 21 −277.004238298AS −15.426909916 CAF2HL −1.55840983 95.944 22 −460.130899964 −74.636127671 −1.00000000 94.984 23 −158.318468619 −30.586960517 CAF2HL −1.55840983 91.216 24 −162.867000225 −41.086604589 −1.00000000 84.569 25 419.508310212 −20.539965049 CAF2HL −1.55840983 83.832 26 −238.581080262 −32.443299462 −1.00000000 84.836 27 −430.197019246 −30.182066783 CAF2HL −1.55840983 92.223 28 691.939037816AS −22.851030925 −1.00000000 93.515 29 −241.462660758AS −10.000000000 CAF2HL −1.55840983 97.602 30 −182.472613831 −25.705407401 −1.00000000 96.085 31 −420.041190250 −36.705938298 CAF2HL −1.55840983 98.486 32 324.867666879 −7.220642187 −1.00000000 99.044 33 −149.830817441 −28.311419778 CAF2HL −1.55840983 94.165 34 −315.631878253AS −11.206528270 −1.00000000 91.678 35 0.000000000 −7.539660426 −1.00000000 92.142 36 −172.862510793 −12.271843841 CAF2HL −1.55840983 88.327 37 −115.635345524 −27.665363620 −1.00000000 82.122 38 −229.213645994AS −32.436472831 CAF2HL −1.55840983 82.891 39 474.721571790 −3.783646156 −1.00000000 82.256 40 −152.435372054 −30.802088433 CAF2HL −1.55840983 76.122 41 −530.778945822 −8.330902516 −1.00000000 71.200 42 −159.504999222 −41.060952888 CAF2HL −1.55840983 63.821 43 3040.455878600 −4.484154484 −1.00000000 51.982 44 −226.630329417AS −24.123224774 CAF2HL −1.55840983 44.183 45 897.778633917 −0.971829936 −1.00000000 33.797 46 0.000000000 −9.700651756 CAF2HL −1.55840983 31.743 47 0.000000000 −7.828847134 −1.00000000 26.288 48 0.000000000 0.000446630 −1.00000000 17.067 ASPHERIC CONSTANTS SURFACE NO. 6 K 0.0000 C1 3.87858881e−009 C2 −1.57703627e−013 C3 1.62703226e−017 C4 −1.12332671e−021 C5 −1.51356191e−026 C6 8.57130323e−031 SURFACE NO. 12 K 0.0000 C1 3.87858881e−009 C2 −1.57703627e−013 C3 1.62703226e−017 C4 −1.12332671e−021 C5 −1.51356191e−026 C6 8.57130323e−031 SURFACE NO. 19 K 0.0000 C1 3.62918557e−009 C2 6.75596543e−014 C3 5.68408321e−019 C4 −6.78832654e−023 C5 6.78338885e−027 C6 −2.05303753e−031 SURFACE NO. 21 K 0.0000 C1 1.19759751e−008 C2 7.35438590e−014 C3 7.03292772e−019 C4 −1.26321026e−023 C5 −3.01047364e−027 C6 2.08735313e−031 SURFACE NO. 28 K 0.0000 C1 −8.39294529e−009 C2 −3.39607506e−013 C3 8.76320979e−018 C4 −1.43578199e−021 C5 5.59234999e−026 C6 2.01810948e−030 SURFACE NO. 29 K 0.0000 C1 1.74092829e−008 C2 −1.69607632e−013 C3 1.18281063e−017 C4 −3.08190938e−021 C5 1.70082968e−025 C6 −1.68479126e−030 SURFACE NO. 34 K 0.0000 C1 −2.14453018e−008 C2 6.73947641e−013 C3 −4.84677574e−017 C4 5.99264335e−021 C5 −2.87629386e−025 C6 3.90592520e−031 SURFACE NO. 38 K 0.0000 C1 1.60415031e−008 C2 4.78837509e−015 C3 2.08320399e−016 C4 −2.87713700e−020 C5 1.77485272e−024 C6 −1.93501550e−029 SURFACE NO. 44 K 0.0000 C1 −6.56394686e−008 C2 −8.25210588e−012 C3 −1.27328625e−016 C4 −1.16616292e−020 C5 −1.58133131e−023 C6 6.39526832e−027

TABLE 5 j33o REFRACTIVE INDEX ½ FREE SURFACE RADII THICKNESSES LENSES 157.63 nm DIAMETER  0 0.000000000 38.054423655 1.00000000 85.333  1 304.292982078 22.168809366 CAF2HL 1.55840983 92.441  2 2741.794481050 96.128678854 1.00000000 92.171  3 0.000000000 0.000000000 −1.00000000 131.865 REFL  4 0.000000000 −467.749539716 −1.00000000 90.082  5 199.893955036 −10.268444544 CAF2HL −1.55840983 91.444  6 486.702942680AS −25.540971142 −1.00000000 96.627  7 186.738998389 −10.064297945 CAF2HL −1.55840983 98.903  8 447.975139348 −19.398954786 −1.00000000 110.873  9 243.529966034 19.398954786 1.00000000 114.137 REFL 10 447.975139348 10.064297945 CAF2HL 1.55840983 111.985 11 186.738998389 25.540971142 1.00000000 102.576 12 486.702942680AS 10.268444544 CAF2HL 1.55840983 101.403 13 199.893955036 465.154328539 1.00000000 96.394 14 0.000000000 0.000000000 −1.00000000 115.447 REFL 15 0.000000000 −100.235657635 −1.00000000 92.750 16 −536.442986965 −25.379215206 CAF2HL −1.55840983 94.346 17 629.049380815 −8.324209221 −1.00000000 93.829 18 0.000000000 −117.663111488 −1.00000000 91.238 19 −312.177007433AS −24.720749191 CAF2HL −1.55840983 94.838 20 −734.696609024 −220.431435837 −1.00000000 94.085 21 −277.004238298AS −15.426909916 CAF2HL −1.55840983 96.283 22 −460.130899964 −74.271177440 −1.00000000 95.326 23 −158.318468619 −30.586960517 CAF2HL −1.55840983 91.580 24 −162.867000225 −41.410948173 −1.00000000 84.915 25 419.508310212 −20.539965049 CAF2HL −1.55840983 84.171 26 −238.581080262 −32.165915708 −1.00000000 85.183 27 −430.197019246 −30.182066783 CAF2HL −1.55840983 92.511 28 691.939037816AS −23.123455275 −1.00000000 93.802 29 −241.462660758AS −10.000000000 CAF2HL −1.55840983 97.962 30 −182.472613831 −25.738903727 −1.00000000 96.437 31 −420.041190250 −36.705938298 CAF2HL −1.55840983 98.835 32 324.867666879AS −7.314163393 −1.00000000 99.389 33 −149.830817441 −28.311419778 CAF2HL −1.55840983 94.515 34 −315.631878253AS −15.768661491 −1.00000000 91.448 35 0.000000000 −3.044279163 −1.00000000 91.163 36 −172.862510793 −12.271843841 CAF2HL −1.55840983 87.933 37 −115.635345524 −27.331297691 −1.00000000 81.792 38 −229.213645994AS −32.436472831 CAF2HL −1.55840983 82.538 39 474.721571790 −4.085179748 −1.00000000 81.887 40 −152.435372054 −30.802088433 CAF2HL −1.55840983 75.743 41 −530.778945822 −8.090865960 −1.00000000 70.786 42 −159.504999222 −41.060952888 CAF2HL −1.55840983 63.559 43 3040.455878600 −4.476231798 −1.00000000 51.715 44 −226.630329417AS −24.123224774 CAF2HL −1.55840983 44.004 45 897.778633917 −0.971829936 −1.00000000 33.650 46 0.000000000 −9.798128149 CAF2HL −1.55840983 31.626 47 0.000000000 0.000000000 IMMERS −1.39000000 26.153 48 0.000000000 −7.818040520 −1.00000000 26.153 49 0.000000000 0.000266950 −1.00000000 17.067 ASPHERIC CONSTANTS SURFACE NO. 6 K 0.0000 C1 3.87858881e−009 C2 −1.57703627e−013 C3 1.62703226e−017 C4 −1.12332671e−021 C5 −1.51356191e−026 C6 8.57130323e−031 SURFACE NO. 12 K 0.0000 C1 3.87858881e−009 C2 −1.57703627e−013 C3 1.62703226e−017 C4 −1.12332671e−021 C5 −1.51356191e−026 C6 8.57130323e−031 SURFACE NO. 19 K 0.0000 C1 3.62918557e−009 C2 6.75596543e−014 C3 5.68408321e−019 C4 −6.78832654e−023 C5 6.78338885e−027 C6 −2.05303753e−031 SURFACE NO. 21 K 0.0000 C1 1.19759751e−008 C2 7.35438590e−014 C3 7.03292772e−019 C4 −1.26321026e−023 C5 −3.01047364e−027 C6 2.08735313e−031 SURFACE NO. 28 K 0.0000 C1 −8.39294529e−009 C2 −3.39607506e−013 C3 8.76320979e−018 C4 −1.43578199e−021 C5 5.59234999e−026 C6 2.01810948e−030 SURFACE NO. 29 K 0.0000 C1 1.74092829e−008 C2 −1.69607632e−013 C3 1.18281063e−017 C4 −3.08190938e−021 C5 1.70082968e−025 C6 −1.68479126e−030 SURFACE NO. 32 K 0.0000 C1 −3.60582630e−011 C2 2.95599027e−015 C3 −7.37891981e−019 C4 6.32721261e−023 C5 −3.13935388e−027 C6 0.00000000e+000 SURFACE NO. 34 K 0.0000 C1 −2.14453018e−008 C2 6.73947641e−013 C3 −4.84677574e−017 C4 5.99264335e−021 C5 −2.87629386e−025 C6 3.90592520e−031 SURFACE NO. 38 K 0.0000 C1 1.60415031e−008 C2 4.78837509e−015 C3 2.08320399e−016 C4 −2.87713700e−020 C5 1.77485272e−024 C6 −1.93501550e−029 SURFACE NO. 44 K 0.0000 C1 −6.56394686e−008 C2 −8.25210588e−012 C3 −1.27328625e−016 C4 −1.16616292e−020 C5 −1.58133131e−023 C6 6.39526832e−027

FIG. 11 shows by way of example a purely refractive, rotationally symmetrical projection objective 40 for high-resolution microlithography, in particular in the DUV wavelength region. In FIG. 11, 41 designates the optical axis of the projection objective 40, 42 denotes the object plane, 43 denotes the image plane, 44 denotes the first group of optical elements, 45 denotes the last optical element, and 46 denotes the immersion medium. Table 1 (appended) summarizes the specification of the design of the projection objective 40 in tabular form. In this case, column 1 specifies the number of refractive surfaces or surfaces otherwise distinguished, column 2 specifies the radius of the surfaces (in mm), column 3 specifies the distance, designated as thickness, of the surface from the subsequent surface (in mm), column 4 specifies the material, column 5 specifies the refractive index of the material at the operating wavelength, and column 6 specifies the maximum useful radius (half the free diameter). The total length L between the object plane and image plane is approximately 1.166 mm. All curvatures are spherical. FIG. 11 shows the projection objective 40 in immersion configuration, and the data in table 1 likewise correspond to the immersion configuration. Table 2 contains the data of the projection objective in dry configuration.

FIG. 12 illustrates a catadioptric projection objective 50 in the case of which the present invention can likewise be used. The catadioptric projection objective 50 with geometric beam splitter 52 is provided for the purpose of imaging a pattern lying in its object plane 53 into the image plane 56 to the scale 4:1 while producing a real intermediate image 54 in the image plane 56. The optical axis 58 is folded at the geometric beam splitter 52 in order to be able to make use when imaging of a concave mirror 60 that facilitates the chromatic correction of the overall system. FIG. 12 and table 3 reproduce the properties of the projection objective 50 in the immersion configuration. Table 4 contains the data of the corresponding dry configuration.

The data of the projection objective 50 are listed in table 5, the surface 32 being formed by a nanosphere. 

1. (canceled)
 2. A system, comprising: a projection objective configured to image radiation from an object plane to an image plane along a radiation path, the projection objective having an optical axis, the projection objective comprising: a plurality of optical elements along the optical axis of the projection objective, the plurality of optical elements comprising a last optical element and a penultimate optical element, the last optical element being the optical element of the plurality of optical elements which is closest to the image plane along the radiation path, and the penultimate optical element being the optical element of the plurality of optical elements which is second closest to the image plane along the radiation path, wherein a distance between the last optical element and the penultimate optical element is variable, and the projection objective is a microlithography projection objective.
 3. The system of claim 2, wherein the distance between the last optical element and the penultimate optical element is variable along the optical axis of the projection objective.
 4. The system of claim 2, further comprising a positioning device configured to move at least one of the plurality of optical elements relative to the image plane.
 5. The system of claim 4, wherein the positioning device is configured to move the last optical element relative to the image plane.
 6. The system of claim 2, further comprising a positioning device configured to move at least one of the optical elements along the optical axis of the projection objective.
 7. The system of claim 6, wherein the positioning device is configured to move the last optical element along the optical axis of the projection objective.
 8. The system of claim 2, further comprising an immersion medium between the last optical element and the image plane.
 9. The system of claim 8, further comprising a positioning device configured to move at least one of the plurality of optical elements relative to the image plane, wherein the positioning device is configured to vary the distance between the last optical element and the penultimate optical element along the radiation path to compensate for temperature variations of the immersion medium or of the environment.
 10. The system of claim 2, wherein at least one of the plurality of optical elements is exchangeable.
 11. The system of claim 2, wherein the last optical element is exchangeable.
 12. The system of claim 2, wherein the system is configured so that, during use of the projection objective, aberrations in the projection objective can be compensated by optical elements other than the last optical element.
 13. The system of claim 2, wherein at least one of the optical elements has an aspheric surface.
 14. The system of claim 2, wherein at least one of the optical elements has a surface with a surface curvature that can be varied.
 15. The system of claim 2, wherein the last optical element is displaceable.
 16. The system of claim 2, wherein the last optical element is displaceable along the optical axis of the projection objective.
 17. The system of claim 2, wherein the projection objective is a catadioptric projection projective.
 18. The system of claim 2, wherein the projection objective has at least one real intermediate image.
 19. The system of claim 2, wherein the last optical element comprises a number of mutually detachable components that are spaced from one another or that are interconnected in an optically neutral fashion.
 20. The system of claim 19, wherein components of the last optical element comprise different optical materials.
 21. The system of claim 20, wherein at least one component of the last optical element comprises fluoride crystal.
 22. A projection exposure machine, comprising: a projection objective configured to image radiation from an object plane to an image plane along a radiation path, the projection objective having an optical axis, the projection objective comprising: a plurality of optical elements along the optical axis of the projection objective, the plurality of optical elements comprising a last optical element and a penultimate optical element, the last optical element being the optical element of the plurality of optical elements which is closest to the image plane along the radiation path, and the penultimate optical element being the optical element of the plurality of optical elements which is second closest to the image plane along the radiation path, wherein a distance between the last optical element and the penultimate optical element is variable, and the projection exposure machine is a microlithography projection exposure machine.
 23. A method, comprising: using a projection exposure machine to produce semiconductor components, the projection exposure machine comprising a projection objective configured to image radiation from an object plane to an image plane along a radiation path, the projection objective having an optical axis, the projection objective comprising: a plurality of optical elements along the optical axis of the projection objective, the plurality of optical elements comprising a last optical element and a penultimate optical element, the last optical element being the optical element of the plurality of optical elements which is closest to the image plane along the radiation path, and the penultimate optical element being the optical element of the plurality of optical elements which is second closest to the image plane along the radiation path, wherein a distance between the last optical element and the penultimate optical element is variable. 